The subject matter of this invention is related generally to molded case circuit breakers and more particularly to the tripping mechanism for molded case circuit breaker apparatus.
Circuit breaker apparatus is taught in U.S. Pat. No. 4,116,205, issued Aug. 28, 1979 to Maier et al and U.S. Pat. No. 3,863,042, issued Jan. 28, 1975 to R. Nicol. In general, a rotatable trip bar is provided in each case for initiating a tripping operation in the circuit breaker in response to either an electrothermal stimulus or an electromagentic stimulus. The electrothermal stimulus is related to I.sup.2 t=K, or said in another way, the amount of overload current present over a predetermined period of time. The electromagentic stimulus is related to short circuit conditions, sometimes referred to as an instantaneous tripping situation. Generally, the calibration of the electrothermal stimulus is related to the angular swing through which the trip bar rotates in response to impingement thereon by a bimetallic member. On the other hand, response to the short circuit condition is related to how quickly an armature can be attracted to an electromagnetic member. In each case, the current flowing in the main terminals of the circuit breaker provides input for the electrothermal or electromagnetic response. As the size of the circuit breaker apparatus is reduced during miniaturization thereof, the need for a highly calibrated and repeatable electrothermal movement requires the continued use of a relatively high angular swing. However, quick electromagnetic response requires a minimum air gap. It is desirable to utilize each of these functions however, it can be seen that the two functions begin to work against each other. That is if the angular movement for thermal response is kept high for calibration repeatability the air gap remains necessarily large and undesirable. On the other hand if the air gap is reduced for the armature of the magnet the angular swing is correspondingly reduced. It would be desirable therefore if the angular swing associated with electrothermal reaction could be kept large and the air gap associated with an electromagnetic reaction could be kept small without affecting the calibration of the electrothermal operation.